This invention relates to a self-emulsifiable paper size composition characterized by improved shelf life and to a method for sizing paper and paperboard therewith. More particularly, the invention relates to a self-emulsifiable paper size composition comprising a mixture of a hydrophobic substituted cyclic dicarboxylic acid anhydride and an ethoxylated castor oil.
Paper and paperboard are often sized with various hydrophobic materials including, for example, rosin, wax emulsions, mixtures of rosin waxes, ketene dimers, isocyanate derivates, fatty acid complexes, fluorocarbons, certain styrene-maleic anhydride copolymers, as well as the substituted cyclic dicarboxylic acid anhydrides more particularly described hereinafter. These sizes may be introduced during the actual paper making operation wherein the process is known as internal or engine sizing, or they may be applied to the surface of the finished web or sheet in which case the process is known as external or surface sizing.
In order to obtain good sizing with any of these sizing compounds, it is desirable that they be uniformly dispersed throughout the fiber slurry in a small particle size. It was general practice therefore, to add the sizes in the form of an aqueous emulsion prepared with the aid of emulsifying agents including, for example, cationic or ordinary starches, carboxymethyl cellulose, natural gums, gelatin, cationic polymers or polyvinyl alcohol, all of which act as protective colloids. The use of such emulsifying agents with or without added surfactants did, however, suffer from several inherent deficiencies in commercial practice. A primary deficiency concerned the necessity of utilizing relatively complex, expensive and heavy equipment capable of exerting high homogenizing shear and/or pressures, together with rigid procedures regarding emulsifying proportions and temperatures, etc., for producing a satisfactory stable emulsion of the particular size. Additionally, the use of many surfactants in conjunction with protective colloids was found to create operational problems in the paper making process such as severe foaming of the stock and/or loss in sizing.
With particular reference to the procedures of the prior art which utilized substituted cyclic dicarboxylic acid anhydrides as sizing agents, it was necessary in commercial practice to pre-emulsify with cationic starch and/or other hydrocolloids using relatively rigid procedures with elevated temperatures to cook the starch or hydrocolloids and high shearing and/or high pressure homogenizing equipment. Unless these complicated procedures were carefully followed, difficulties such as deposition in the paper system, quality control problems and generally unsatisfactory performance were often encountered.
Many of these problems were overcome in U.S. Reissue Pat. No. 29,960 which disclosed the use of a size mixture of these cyclic dicarboxylic acid anhydrides and specific polyoxyalkylene alkyl or alkyl-aryl ethers or their corresponding mono- or di-esters, which mixture was easily emulsifiable with water in the absence of high shearing forces and under normal pressure by merely stirring, passing through a mixing valve or common aspirator or by the usual agitation present in a stock preparation system. While satisfactory sizing properties for commercial uses were achieved with this method, the shelf stability of the mixture was poor and it was necessary to use the size mixture immediately after preparation thereof. The two components were therefore supplied separately to the paper manufacturer who continued the separate storage of the components until their use was required.
It would be desirable to be able to prepare a size in the form of a shelf stable mixture which could be stored in warehouses for periods of six months to a year, which would be self-emulsifiable and would exhibit excellent sizing properties.